SQL 如何从连接表中的多个值 select

Question

I have two tables A and B joined with a common id field. Table A has single entries per id. Table B has multiple entries per id. Table B has two columns "name" and "customertype". I want to be able to SELECT columns from A and B but I want to show the column "name" from Table B three times: once for each separate match of customertype.

Example:

I want to be able to SELECT and display:

id,country,name(from EC),date,name(from T1),name(from T2)

How do I do this? I am a novice SQL user. Thanks in advance for advice.

Answer 1

You can use conditional aggregation:

select a.id, a.country, a.date,
       max(case when customertype = 'EC' then name end) as ec,
       max(case when customertype = 'T1' then name end) as t1,
       max(case when customertype = 'T2' then name end) as et2
from a join
     b
     on a.id = b.id
group by a.id, a.country, a.date

Answer 2

You can join and use conditional aggregation:

select a.id, a.country, a.date,
    max(case when b.customertype = 'EC' then name end) name_ec,
    max(case when b.customertype = 'T1' then name end) name_t1,
    max(case when b.customertype = 'T2' then name end) name_t2
from tablea a 
inner join tableb b on b.id = a.id
group by a.id, a.country, a.date